HERMOSA BEACH, Calif. A man who says he once babysat for Newtown, Conn., gunman Adam Lanza says he recalls Lanza's mother warning him never to turn his back on the boy - not even to go to the bathroom.

Ryan Kraft now lives in Hermosa Beach, in Southern California.

But, he tells CBS station KCBS in Los Angeles, he was once a student at Sandy Hook Elementary School in Newtown, Conn., and babysat for Lanza when Lanza was about 9 or 10 and Kraft was 14 or 15.

Police say Lanza, 20, went on a rampage in the school Friday, killing 20 six- and seven-year-olds and six adults before taking his own life. Lanza also shot his mother, Nancy Lanza, to death in the nearby home they shared before heading to the school, authorities say.

Kraft tells KCBS when he first heard about the shooting and that Lanza was involved, "I just couldn't think for a little while. I was shaking."

He says he recalls Nancy Lanza cautioning him never to turn his back on Adam -- "to keep an eye on him at all times ... to never turn my back, or even to go to the bathroom or anything like that."

Kraft says he remembers Lanza as quiet, very intelligent and introverted, noting, "Whenever we were doing something, whether it was building Legos, or playing video games, he was really focused on it. It was like he was in his own world."

Kraft is still having trouble believing the kid he babysat could have been involved in such unspeakable horror. "I'm just numb to it, I haven't really processed the fact that this happened right where I used to be, and that, 15 years ago, it could have been me."